1. Field of the Invention
The present invention concerns a diaphragm for a specific manipulation (modification) of x-ray radiation that emanates from an x-ray focus, such as a CT apparatus, and that serves for scanning an examination subject, of the type wherein the x-ray focus and the diaphragm arranged relatively closer to the focus can be rotated together around a system axis (z-axis) and the diaphragm has movable diaphragm elements by means of which a diaphragm aperture—and therefore the spatial divergence of the x-rays passing through the diaphragm aperture—is dynamically adjustable. Furthermore, the invention concerns a diaphragm device and a CT apparatus, with such a diaphragm.
2. Description of the Prior Art
A CT apparatus for scanning an examination subject has, as is known at least one x-ray source with an x-ray focus from which a pyramid- or fan-shaped x-ray beam is directed through the examination subject (for example a patient) onto a detector system made up of multiple detector elements. The fan or pyramid shape of the x-ray beam is thereby typically matched to the detector system that is used so that the detector system is always completely exposed by the x-ray beam. The measurement area (“field of view”, FOV) of the x-ray tube detector system is therefore also established. Depending on the design of the CT apparatus, the x-ray source and the detector system are mounted, for example, on a gantry or a C-arm that can rotate around a system axis (z-axis). Furthermore, a positioning device for the examination subject is provided that can be moved or displaced along the system axis (z-axis).
During the CT acquisition, each of the detector elements of the detector system that is struck by the x-ray radiation produces a signal that represents a measure of the total transparency of the examination subject for the radiation emanating from the radiation source on its path to the detector system, i.e., it represents the radiation attenuation. The set of output signals of the detector elements of the detector system that is acquired for a specific position of the radiation source is designated as a projection. The position of the x-ray focus from which the x-ray beam emanates and penetrates the examination subject is continuously varied as a result of the rotation of the gantry/the C-arm. The current position of the x-ray focus or of the detector system can be specified in polar coordinates (r, z, φ) for a given system axis (z-axis). During operation, the r-coordinates of the radiation source and of the associated detector system are typically constant. A scan of the examination subject thus includes a number of projections that have been respectively acquired at different positions of the gantry of the C-arm and/or at the various positions of the positioning device. A differentiation is made between sequential scanning methods and spiral scan methods. Based on the (projection) measurement data generated in a scan, 2D or 3D image data of the examination subject can be reconstructed by means of known methods.
Conventionally, the examination subject is scanned utilizing the FOV of the x-ray tube detector system. Particularly in examinations in which only a small volume section (ROI, “region of interest”) of the examination subject is of interest, this leads to an exposure of an examination subject volume that is markedly larger in comparison to the ROI. The examination subject (for example a patient) is subjected unnecessarily to high radiation exposure since the examination subject is exposed in the entire measurement field of the x-ray tube detector system (FOV), but the actual ROI is significantly smaller by many times than the FOV.
To avoid an unnecessary radiation exposure given a scan of an examination subject with a CT apparatus, from DE 102 42 920 A1 a diaphragm device is known with which the beam can be adjusted in a very precise manner on the measurement field of the detector. The diaphragm device has a diaphragm with two radiator-side absorber elements that adjust the x-ray beam emanating from the x-ray focus. The diaphragm is fashioned so that the two absorber elements can be positioned with a high positioning precision before the beginning of an examination.
A CT apparatus with two diaphragms arranged in series is known from DE 196 25 864 C2, which diaphragms serve to gate a pyramidal x-ray beam. Only the diaphragm situated farther away from the focus is adjustable.
A primary beam diaphragm is known from DE 42 29 321 A1 in which two diaphragm pairs can each be adjusted in opposite directions in two parallel planes so that a rectangular gating of the x-ray beam is possible.
Moreover, from DE 31 36806 A1 an x-ray examination apparatus is disclosed which has an x-ray tube with different adjustable focal spots and an adjustable diaphragm. Upon switching the focal spot, the diaphragm can be displaced synchronously in the same direction and by approximately the same amount by which the focal spot is displaced.
Furthermore, from DE 10 2005 018 811 B4 a diaphragm device is known in which at least two diaphragms are provided for beam shaping during the scanning of an examination subject, wherein for at least one segment of the scan a beam adjusted with the first diaphragm can be at least partially dynamically masked out by means of the second diaphragm, and wherein the adjustment of the beam by the first diaphragm can be implemented with a high positional accuracy and the dynamic masking by the second diaphragm can be implemented with a high positioning speed.
Some of the diaphragms or diaphragm systems cited above allow a specific scanning of a predetermined ROI in an examination subject. With the known diaphragms or the diaphragm systems, a dynamic limitation of the spatial divergence of the x-ray beam used for scanning the examination subject ensues at the ROI. X-rays that are directed to regions outside of the ROI are thereby masked out. A marked reduction of the radiation dose acting on the examination subject (on the order of 50%) can be achieved. However, for an artifact-free image reconstruction of the projection data that are acquired using such known diaphragms or systems, the problem exists that projection data outside of the ROI are also required, at least for folding within the scope of filtered back-projection. This problem is presently solved by implementing a scan with a full measurement field (FOV) of the x-ray tube detector system before the implementation of the dynamic scan that is limited to an ROI. From the scan with the full measurement field (FOV), the necessary projection data for the image reconstruction with the needed supplementary data are available for all later dynamic scans. However, this assumes that nothing changes at the position of the ROI during the entire dynamic scan. If the examination subject (for example a patient) moves during the examination, the data supplementation from the pre-scan is incorrect and leads to image artifacts.